The present invention relates to an electromagnetic clutch and relates particularly although not exclusively to an electromagnetic microclutches for use in automation appliances.
FIG. 1 illustrates a typical example of the conventional electromagnetic microclutch. This electromagnetic clutch has a hollow cylindrical rotor shaft 2 formed of a synthetic resin by molding. The rotor shaft 2 has a rotor 3, mounted around on it, and an annular yoke 4 rotatably mounted on it so as to be positioned on the left side in FIG. 1 with respect to the rotor 3. The rotor 3 has a boss 5 and a cup-shaped flange portion 6 concentrically and integrally formed with the boss 5, the flange portion 6 having an outer circumferential magnetic pole 6a and an inner circumferential magnetic pole 6b. The yoke 4 has a coil 7 and a yoke locking plate 8 welded to it. The yoke locking plate 8 is provided in its distal end with an engaging recess 8a into which a lock pin 9 fits. The lock pin 9 is attached at its proximal end to a frame 10 for keeping the locking plate 8 stationary, so that the yoke 4 is prevented from accompanying the rotor shaft 2 when the latter is rotated. A spur gear 11 rotatably fits around the rotor shaft 2 and is prevented from axial movement by both the boss 5 of the rotor 3 and a retaining ring 12 mounted to the rotor shaft 2. A ring-shaped armature 13 is fixed to the spur gear 11 through a ring-shaped spring plate 14 so that one face thereof faces to the inner and outer magnetic poles 6b and 6a with a small gap.
With such a construction, the armature 13 is rotated together with the spur gear 11 when a rotation force is transmitted from a drive mechanism (not shown), including an electric motor, to the spur gear 11. During the rotation of the armature 13, the coil 7 is energized for magnetizing the yoke 4, so that a magnetic flux passes the yoke 4, the outer circumferential magnetic pole 6a of the rotor 3, the armature 13 and the inner circumferential magnetic pole 6b and then returns to the yoke 4, thus forming a magnetic path .phi..sub.1. The armature 13 is hence brought into contact with the magnetic poles 6a and 6b by magnetic attraction against the resilient force exerted by the spring plate 14, so that the rotation shaft 2 is rotated. When the coil 7 is deenergized, the armature 13 is separated from the rotor 3 by the resilient force of the spring plate 14, with the result that transmission of the torque from the spur gear 11 to the rotation shaft 2 is discontinued.
Such an electromagnetic clutch has the following problems in size reduction:
(a) Reduction in the outer diameter of the electromagnetic clutch decreases the space into which the coil 7 is received. This results in considerable reduction in the magnetomotive force of the coil 7.
(b) The rotor shaft 2 is molded of a synthetic resin in view of its rather complicated configuration and for cost reduction. Thus, the radial cross-sectional area of the magnetic path of the rotor shaft becomes smaller than in a magnetic rotor shaft for a given radial cross-sectional area of an electromagnetic clutch. This results in an increase in magnetic reluctance of the magnetic path .phi..sub.1 and hence in necessity for a larger magnetomotive force of the coil.
(c) The synthetic resin rotor shaft is a hollow shaft into which a driven shaft having a predetermined outer diameter is inserted for interconnection and thus it has a lower limit in its thickness in view of strength, so that the radial size reduction of the electromagnetic clutch is rather limited. When the rotor shaft is made of an iron material, its machining cost is rather high as compared to that of the synthetic resin rotor shaft and thus considerably raises the production cost of the electromagnetic clutch.
Accordingly, it is an object of the present invention to provide an electromagnetic clutch which enables its size reduction with relatively small cost.